1. Field of the Invention
The present invention relates generally to a new interconnection and a method for making and detaching the same, and more particularly, to a solder connection. This invention addresses the card assembly/rework issues associated with column and ball grid array (CGA and BGA) structures by introducing an interconnection structure with a transient liquid solder interface that includes fine metal powder additives.
2. Description of Related Art
Ceramic column grid array (CGA) packages are being used for many high performance ASIC chips and microprocessor chips for IBM and original engineering manufacturer (OEM) customers. The preferred processes for lowest cost manufacturing and minimum handling damage during bond, assembly, and test are the wire CGA process and the ball grid array (BGA) process. However, inherent to the joining process of the wire CGA and BGA structure is an eutectic interface used in the column or ball attachment to the substrate pads. As a result, during circuit card joining and reworking of these packages, two problems are typically experienced. First, during joining, the solder composition attaching the columns to the substrate can melt causing the columns to tilt, thus, putting the interconnect structure out of alignment. Second, during rework to remove the CGA or BGA, columns or balls may be left on the circuit card. Currently, since the columns and balls are made of 90/10 Pb/Sn solder, this eutectic material residue remaining on the chip terminals and/or testing substrate pads must be individually removed. This is a labor intensive process that also critically increases the length of time that the card site must be kept within the temperature range necessary to facilitate removal of all the columns or balls (200-220xc2x0 C.). Typically, a hand held hot gas tool with an integral vacuum nozzle is moved back and forth over the card site. The hot gas melts the eutectic solder on the circuit card lands, and the vacuum picks up the balls or columns from the lands. This process may be repeated, especially with columns which tilt or detach from the substrate during the vacuum xe2x80x9csweepxe2x80x9d operation. Repeated localized heating to pick up columns has led to circuit card land delaminations causing the entire card to be unusable. Additionally, this problem will further develop as package sizes increase, and more columns or balls are added to the structure.
The prior art has dealt with these deficiencies in a number of diverse ways. In U.S. Pat. No. 5,130,779, issued to Agarwala et al., on Jul. 14, 1992, entitled, xe2x80x9cSOLDER MASS HAVING CONDUCTIVE ENCAPSULATING ARRANGEMENTxe2x80x9d, an elongated solder interconnection is formed on an electronic carrier by capping a metal layer on a deposited solder mass. A further elongated solder interconnection is then formed by the addition of a second solder mass on the first solder mass capped with the metal layer. The melting point of the subsequent solder mass may be different than the first solder mass to facilitate component removal. The different melting points allow the low-melt solder to reflow to its mating substrate pad at a temperature lower than the high-melt solder""s melting point. The primary purpose of the metal barrier layer is to prevent the migration of the intermediate melting point solder towards the low melt solder. Thus, while the low-melt solder flows, the high-melt solder remains in the solid state, as it does prior to the reflow cycle. Upon reflow, the encapsulated, high-melt solder on the electronic components and the low-melt solder deposited over this metal barrier layer forms a new solder mass having an intermediate melting point, which is higher than the melting point of the low-melt solder mass on the other side of the barrier layer. Thus, the electronic components can later be separated, as the low-melt solder mass would melt prior to the intermediate melting point solder mass. This prior art patent, however, does not address creating a transient liquid solder interface with fine metal powder additives, which after reflow and attachment to the substrate pads create a higher effective melting point interface.
In U.S. Pat. No. 5,326,016, issued to Cohen et al. on Jul. 5, 1994, entitled, xe2x80x9cMETHOD FOR REMOVING ELECTRICAL COMPONENTS FROM PRINTED CIRCUIT BOARDSxe2x80x9d, the leads of each component are connected to circuitry on the circuit board by a connection alloy comprised of two constituent metals and having a given melting point less than that of either of the constituent metals A removal alloy, having a particular melting point substantially below the given melting point of the connection alloy, and typically in the form of a wire, is heated to a temperature greater than its particular melting point but below the given melting point of the connection alloy. Next, the connection alloy on each of the leads of the integrated circuit are contacted by the molten removal alloy causing a reaction that also produces a molten state for the connection alloy. After the connection alloy has reached a molten state, the integrated circuit is safely separated from the circuit board. This process, however, does not utilize a metal powder mixed in the interface solder, balls and columns, being joined to the substrate prior to any card attachment or rework. Also, this process lowers the melting point of the interconnection joint to allow removal. In contrast, metal powder additions effectively increase the melting point of the interface, instead of lowering it. Metal powder additions do not melt during ball or column attachment to the substrate, or during card assembly or rework.
In U.S. Pat. No. 5,234,149, issued to Katz et al. on Aug. 10, 1993, entitled, xe2x80x9cDEBONDABLE METALLIC BONDING METHODxe2x80x9d, a method of joining a device or carrier to another substrate is taught, providing a bondable first metallurgy on the device side for the solder connections (ball) and providing a second metallurgy on the substrate side to which these solder balls can make electrical contact under pressure, but not wet, to create a permanent metallurgical bond. The solder ball is heated to a temperature that facilitates wetting the first but not the second metallization, whereby the device can be mechanically pulled away from the substrate while the melted solder balls are immersed in the liquid flux. However, there is no disclosure regarding changing the interface solder melting point by addition of high melting reacting powders.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an interconnection structure between a device substrate or module assembly and a circuit card.
It is another object of the present invention to provide a low melting point solder layer that allows for an integrated circuit chip to be attached to the next level of packaging at substantially lower temperatures (approximately 100xc2x0 C. less) than would the high melting point solder.
A further object of the present invention is to provide for a number of variations for a transient melting solder comprising a low melting alloy powder/powder mixture and a higher melting alloy additive in the form of a powder.
Another object of the present invention is to minimize the likelihood of handling damage occurring on the relatively fragile interconnect columns.
Still other objects of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, an interconnect structure comprising: a first substrate; a second substrate; a connector for providing electrical and mechanical interconnection between the first and second substrates; a first solder bond at one end of the connector; and, a second solder bond at the opposite end of the connector, the second solder bond having a lower melting point than the first solder bond. The interconnect structure may be an elongated, conductive pin or column, or a conductive ball of substantially spherical shape.
The first solder bond of the interconnect structure comprises a combination of a Pbxe2x80x94Sn eutectic and a Pbxe2x80x94Sn alloy wherein the Pb in the alloy comprises from about 33-50% by weight of the alloy. The total of the Pbxe2x80x94Sn eutectic and the Pbxe2x80x94Sn alloy may be from about 80-98% by weight of the solder bond, or 90-98% by weight of the solder bond, depending upon the metal alloy used. For an 80-98% combination, a Pbxe2x80x94Sn alloy, Ptxe2x80x94Sn alloy, or Cuxe2x80x94Sn alloy would be used in the range of 2-20% by weight. Whereas, for a 90-98% combination, a Pd alloy, a Pt alloy, or a Cu alloy may be used in the range of 2-10% by weight. The second solder bond comprises a Pbxe2x80x94Sn eutectic.
In a second aspect, the invention is directed to a method for forming a connection between substrates comprising: bonding a connector to an electrical conductive portion of a first substrate, with a first solder composition having a first melting point by heating the first solder to a temperature higher than the first melting point; and, bonding an opposite end of the connector to an electrical conductive portion of a second substrate with a second solder composition having a second melting point, wherein the second melting point is lower than the first melting point. This method may be conducted in two ways. First, by applying the first solder composition to the electrical conductive portion of the first substrate by screening mask, such that the first substrate is reflowed, melting the first solder composition and wetting the electrical conductive portion of the first substrate. Or, second, by transferring solder preforms of the first solder composition to the electrical conductive portion of the first substrate by loading the solder preforms in a carrier such that the preforms align with the electrical conductive portion of the first substrate, and heating the preforms above the first melting point of the first solder composition.
In a third aspect, the invention comprises a method for forming an interconnect structure between a substrate with an I/O pad and a circuit card with a conductive land comprising: applying a solder composition of a first melting point to the substrate I/O pad; forming an intermediate assembly by loading the interconnect structure into a carrier wherein the interconnect structure is aligned with the substrate I/O pad and in position to form an array; heating the intermediate assembly to a temperature higher than the first melting point; extracting the substrate from the carrier such that the interconnect structure is bonded to the substrate; cleaning the substrate with the interconnect structures bonded thereto; inserting the substrate with the interconnect structures attached into the circuit card land; and, applying a solder composition of a second melting point to the circuit card land and the interconnect structure, wherein the second melting point is lower than the first melting point such that only the solder composition of the second melting point changes to a molten state.
In a fourth aspect, the present invention comprises a method for detaching two substrates joined by the interconnect structures described above, wherein the connector is bonded to a first substrate at one end with a solder composition of a first melting point, and to a second substrate at opposite end with a solder composition of a second melting point, the first melting point higher than the second melting point, the method comprising: heating the substrates with the connector attached to a temperature higher than the second melting point but lower than the first melting point such that the solder composition of the second melting point reaches a molten state; separating the substrates with the connector attached such that the connector remains attached to the substrate with the solder composition of the first melting point; and, cleaning the substrates of excess solder debris.